The militant group has bombed schools, churches and mosques; kidnapped women and children; and assassinated politicians and religious leaders alike.

One of its most brazen attacks occurred in April, when gunmen abducted more than 200 girls in the town of Chibok. After a fierce gunbattle with soldiers, the militants herded the girls out of bed and onto buses, and sped off. Only a few dozen of the girls have escaped.

What exactly is Boko Haram, and why has it turned into a Nigerian synonym for fear and bloodshed?

Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A video of Abubakar Shekau, who claims to be the leader of the Nigerian Islamist extremist group Boko Haram, is shown in September 2013. Boko Haram is an Islamist militant group waging a campaign of violence in northern Nigeria. The group's ambitions range from the stricter enforcement of Sharia law to the total destruction of the Nigerian state and its government. Click through to see recent bloody incidents in this strife-torn West African nation:

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Boko Haram: Nigeria's crisis 23 photos

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Bodies lie in the streets in Maiduguri, Nigeria, after religious clashes on July 31, 2009. Boko Haram exploded onto the national scene in 2009 when 700 people were killed in widespread clashes across the north between the group and the Nigerian military.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – An official displays burned equipment inside a prison in Bauchi, Nigeria, on September 9, 2010, after the prison was attacked by suspected members of Boko Haram two days earlier. About 720 inmates escaped during the prison break, and police suspect the prison was attacked because it was holding 80 members of the sect.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Nigerian President Goodluck Jonathan, second from left, stands on the back of a vehicle after being sworn-in as President during a ceremony in the capital of Abuja on May 29, 2011. In December 2011, Jonathan declared a state of emergency in parts of the country afflicted by violence from Boko Haram.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Rescue workers help a wounded person from a U.N. building in Abuja, Nigeria, on August 26, 2011. The building was rocked by a bomb that killed at least 23 people, leaving others trapped and causing heavy damage. Boko Haram had claimed responsibility for the attack in which a Honda packed with explosives rammed into the U.N. building, shattering windows and setting the place afire.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A photo taken on November 6, 2011, shows state police headquarters burned by a series of attacks that targeted police stations, mosques and churches in Damaturu, Nigeria, on November 4, 2011. Attackers left scores injured -- probably more than 100 -- in a three-hour rampage, and 63 people died.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Men look at the wreckage of a car after a bomb blast at St. Theresa Catholic Church outside Abuja on December 25, 2011. A string of bombs struck churches in five Nigerian cities, leaving dozens dead and wounded on the Christmas holiday, authorities and witnesses said. Boko Haram's targets included police outposts and churches as well as places associated with "Western influence."

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A paramedic helps a young man as he leaves a hospital in the northern Nigerian city of Kano on January 21, 2012. A spate of bombings and shootings left more than 200 people dead in Nigeria's second-largest city. Three days later, a joint military task force in Nigeria arrested 158 suspected members of Boko Haram.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A photo taken on June 18, 2012, shows a car vandalized after three church bombings and retaliatory attacks in northern Nigeria killed at least 50 people and injured more than 130 others, the Nigerian Red Cross Society said.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A French family kidnapped February 19, 2013, in northern Cameroon is released after two months in captivity in Nigeria. The family of four children, their parents and an uncle were kidnapped in Waza National Park in northern Cameroon, situated near the border with Nigeria. One of the captive men read a statement demanding that Nigeria and Cameroon free jailed members of Boko Haram.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A soldier stands in front of a damaged wall and the body of a prison officer killed during an attack on a prison in the northeastern Nigerian town of Bama on May 7, 2013. Two soldiers were killed during coordinated attacks on multiple targets. Nigeria's military said more than 100 Boko Haram militants carried out the attack.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A deserted student hostel is shown on August 6, 2013, after gunmen stormed a school in Yobe state, killing 20 students and a teacher, state media reported.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A photograph made available by the Nigerian army on August 13, 2013, shows improvised explosive devices, bomb-making materials and detonators seized from a Boko Haram hideout. Gunmen attacked a mosque in Nigeria with automatic weapons on August 11, 2013, killing at least 44 people.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Nigerian students from Jos Polytechnic walk on campus in Jos, Nigeria, on September 30, 2013. Under the cover of darkness, gunmen approached a college dormitory in a rural Nigerian town and opened fire on students who were sleeping. At least 40 students died, according to the News Agency of Nigeria.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Soldiers stand outside the 79 Composite Group Air Force base that was attacked earlier in Maiduguri on December 2. Hundreds of Boko Haram militants attacked an Air Force base and a military checkpoint, according to government officials.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Catholic priest Georges Vandenbeusch speaks to reporters outside Paris after his release on January 1. Vandenbeusch was snatched from his parish church in Cameroon on November 13. Boko Haram claimed responsibility for kidnapping the priest.

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Boko Haram: Nigeria's crisis 23 photos

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – A man receives treatment at Konduga specialist hospital after a gruesome attack on January 26. It was suspected that Boko Haram militants opened fire on a village market and torched homes in the village of Kawuri, killing at least 45 people.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Police officers stand guard in front of the burned remains of homes and businesses in the village of Konduga on February 12. Suspected Boko Haram militants torched houses in the village, killing at least 23 people, according to the governor of Borno state on February 11.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Yobe state Gov. Ibrahim Gaidam, left, looks at the bodies of students inside an ambulance outside a mosque in Damaturu. At least 29 students died in an attack on a federal college in Buni Yadi, near the capital of Yobe state, Nigeria's military said on February 26. Authorities suspect Boko Haram carried out the assault in which several buildings were also torched.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Rescue workers try to put out a fire after a bomb exploded at the busiest roundabout near the crowded Monday Market in Maiduguri on July 1.

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Boko Haram: Nigeria's crisis 23 photos

Boko Haram: Nigeria's crisis – Police in riot gear block a route in Abuja on October 14, during a demonstration calling on the Nigerian government to rescue schoolgirls kidnapped by Boko Haram. In April, more than 200 girls were abducted from their boarding school in northeastern Nigeria, officials and witnesses said.

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What does 'Boko Haram' mean?

The name translates to "Western education is sin" in the local Hausa language.

The militant group says its aim is to impose a stricter enforcement of Sharia law across Africa's most populous nation, which is split between a majority Muslim north and a mostly Christian south.

Bring Back Our Girls 13 photos

Bring Back Our Girls13 photos

'Bring Back Our Girls!' – Weeks after the April 14 kidnapping of more than 200 Nigerian girls, worried families and supporters blamed the government for not doing enough to find them. Their cries spread worldwide on social media under the hashtag #BringBackOurGirls. From regular people to celebrities, here are some of the people participating in the movement.

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'Bring Back Our Girls!' – First Lady Michelle Obama tweeted this picture of herself holding a #BringBackOurGirls sign in support of the schoolgirls abducted by Boko Haram in Nigeria.

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Bring Back Our Girls13 photos

'Bring Back Our Girls!' – "We hear it like it's not happening in our region so it doesn't concern us but that's not right," says Milliscent Maduagwu from Port Harcourt, Nigeria. "This fight on terrorism is not just the Commander in Chief's and the army, but ours as well!"

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'Bring Back Our Girls!' – "These girls could have been my sisters or worst still my daughter," says Emmanuel Oleabhiele from Doha, Qatar. "My daughter is 6 months old and I fear for her future as a Nigerian."

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Bring Back Our Girls13 photos

'Bring Back Our Girls!' – Malala Yousafzai, the world's most famous advocate for girls' right to education, says that "girls in Nigeria are my sisters." This photo was posted to the @MalalaFund Twitter account on May 6.

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Bring Back Our Girls13 photos

'Bring Back Our Girls!' – Ify Elueze of Bonn, Germany, asks, "How can the world sit and watch?! It is no longer just the responsibility of the Nigerian government, now it is your responsibility and mine!"

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Bring Back Our Girls13 photos

'Bring Back Our Girls!' – British supermodel Cara Delevingne posted this photo on her Instagram account saying, "Everyone help and raise awareness #regram #repost or make your own!"

'Bring Back Our Girls!' – American singer-songwriter Alicia Keys posted this photo on her Instagram account with this message: "I'm so saddened and enraged that these girls are not back where they belong! Safe at home and at school! Safe with their families! Safe to become the incredible leaders and powerful voices they are yet to be."

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'Bring Back Our Girls!' – "You are most likely thinking to yourself, 'What's the point in posting another picture or status to speak about this issue?,"' says Uchenna Mildred Udeh from St. John, New Brunswick. "I can tell you this: it will. Do not underestimate the power of your voice. We have to make this personal."

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Bring Back Our Girls13 photos

'Bring Back Our Girls!' – British singer-songwriter Leona Lewis took a stand to #BringBackOurGirls on her Twitter account on May 7.

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Bring Back Our Girls13 photos

'Bring Back Our Girls!' – "I think it's high time we all start praying for Nigeria and stop complaining," says Lotanna Ugwu from Abuja, Nigeria. "It's only God that can touch the hearts of those who kidnapped the young girls."

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Bring Back Our Girls13 photos

Bring Back Our Girls – Singer Steven Tyler posted this image on both his Twitter and Instagram accounts alonth with the hashtag #RealMenDontBuyGirls.

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In recent years, its attacks have intensified in an apparent show of defiance amid the nation's military onslaught. Its ambitions appear to have expanded to the destruction of the Nigerian government.

How long has it been around?

The group was founded 12 years ago by Mohammed Yusuf, a charismatic cleric who called for a pure Islamic state in Nigeria. Police killed him in 2009 in an incident captured on video and posted to the Internet.

The crackdown, some say, made Boko Haram more violent and defiant.

Abubakar Shekau took control of the group and escalated the attacks. It murdered and kidnapped Westerners and started a bombing campaign that targeted churches, mosques and government buildings.

Why not just kill Abubakar Shekau?

One word: elusive.

Questions have swirled about Shekau, including whether he's dead or alive. Even his age is unknown -- estimates range between 35 and 44.

In recent years, the Nigerian military has touted his death, only to retract its claim after he appeared alive and vibrant in propaganda videos.

He uses the alias Darul Tawheed, and analysts describe him as a ruthless loner and master of disguise. He does not speak directly with members, opting to communicate through a few select confidants.

Why would an Islamist militant group target the Muslim north?

Despite its religious fanaticism, Boko Haram does not consider all Muslims as supporters and allies.

There have been suggestions that it attacks certain mosques because members have spoken out against it and helped federal officials with their crackdown. Its attacks are aimed at striking fear at the heart of the local population to prevent cooperation with the government, analysts say.

Does the north support the group?

Although the northern populace mostly abhors the violence, there is considerable local sympathy and support for Sharia law, seen by many as the only way to end what is widely regarded as a corrupt and inept government. Poverty is prevalent in the northern region, and as the military struggles to halt Boko Haram's attacks, the militant group is winning perhaps its most important battle: making Nigerians question government competency.

Rights groups have accused local authorities of human rights violations in the fight against the group, adding to the anti-government sentiment.

What's the West doing to help?

The United States has put a $7 million bounty on Shekau's head. It also designated Boko Haram as a foreign terrorist network last year. Though it has provided technical and financial support to the Nigerian teams battling the insurgency, there has been a reluctance to put boots on the ground unless there's a direct national security threat to the West. Boko Haram's attacks have been limited primarily to Nigeria.

I don't live in Nigeria, so why should I care?

With a population of 175 million, Nigeria is Africa's most populous nation and is considered a political and economic powerhouse in the continent. The key U.S. partner is rich in oil, a major trading partner with China and is the hub of global business in the region.

And as we've learned with Mali, any unresolved local Islamist insurgency has the potential of spiraling into a world problem.

Last year, Shekau released a statement vowing to attack the United States and Europe.

"Our strength and firepower is bigger than that of Nigeria. Nigeria is no longer a big deal to us, as far as we are concerned. We will now comfortably confront the United States of America," he said.

Does it have ties to al Qaeda?

The United States says Boko Haram has links to the al Qaeda affiliate in West Africa and to extremist groups in Mali.

What other attacks has the group conducted?

In April, a massive explosion ripped through a bus station in the Nigerian capital, killing at least 71 people. In a video, Shekau said the group was behind the attack.

In November, the group abducted dozens of Christian women, most of whom were later rescued by the military. Some were pregnant or had children, and others had been forcibly converted to Islam and married off to their kidnappers.

In 2011, a Boko Haram suicide attack on the United Nations building in Abuja killed at least 25 people.